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Tuesday, July 12, 2016

Pulsars could probe black hole horizons

The first antenna of MeerKAT,
a SKA precursor in South Africa.
[Image Source.]

It’s hard to see black holes – after all, their defining feature is that they swallow light. But it’s also hard to discourage scientists from trying to shed light on mysteries. In a recent paper, a group of researchers from Long Island University and Virginia Tech have proposed a new way to probe the near-horizon region of black holes and, potentially, quantum gravitational effects.

The idea is simple and yet promising: Search for a binary system in which a pulsar and a black hole orbit around each other, then analyze the pulsar signal for unusual fluctuations.

A pulsar is a rapidly rotating neutron star that emits a focused beam of electromagnetic radiation. This beam goes into the direction of the poles of the magnetic field, and is normally not aligned with the neutron star’s axis of rotation. The beam therefore spins with a regular period like a lighthouse beacon. If Earth is located within the beam’s reach, our telescopes receive a pulse every time the beam points into our direction.

Pulsar timing can be extremely precise. We know some pulsars that have been flashing for decades every couple of milliseconds to a precision of a few microseconds. This high regularity allows astrophysicists to search for signals which might affect the timing. Fluctuations of space-time itself, for example, would increase the pulsar-timing uncertainty, a method that has been used to derive constraints on the stochastic gravitational wave background. And if a pulsar is in a binary system with a black hole, the pulsar’s signal might scrape by the black hole and thus encode information about the horizon which we can catch on Earth.

No such pulsar-black hole binaries are known to date. But upcoming experiments like eLISA and the Square Kilometer Array (SKA) will almost certainly detect new pulsars. In their paper, the authors estimate that SKA might observe up to 100 new pulsar-black hole binaries, and they put the probability that a newly discovered system would have a suitable orientation at roughly one in a hundred. If they are right, the SKA would have a good chance to find a promising binary.

Much of the paper is dedicated to arguing that the timing accuracy of such a binary pulsar could carry information about quantum gravitational effects. This is not impossible but speculative. Quantum gravitational effects are normally expect to be strong towards the black hole singularity, ie well inside the black hole and hidden from observation. Naïve dimensional estimates reveal that quantum gravity should be unobservably small in the horizon area.

However, this argument has recently been questioned in the aftermath of the firewall controversy surrounding black holes, because one solution to the black hole firewall paradox is that quantum gravitational effects can stretch over much longer distances than the dimensional estimates lead one to expect. Steve Giddings has long been a proponent of such long-distance fluctuations, and scenarios like black hole fuzzballs, or Dvali’s Bose-Einstein Computers also lead to horizon-scale deviations from general relativity. It is hence something that one should definitely look for.

Previous proposals to test the near-horizon geometry were based on measurements of gravitational waves from merger events or the black hole shadow, each of which could reveal deviations from general relativity. However, so far these were quite general ideas lacking quantitative estimates. To my knowledge, this paper is the first to demonstrate that it’s technologically feasible.

Michael Kavic, one of the authors of this paper, will attend our September conference on “Experimental Search for Quantum Gravity.” We’re still planning to life-streaming the talks, so stay tuned and you’ll get a chance to listen in.

You're not cynicaL to think it, because it's plausible for lots of more general reasons, why a quiet aversion to tests should be taking root at the moment, despite tests being also what everyone agrees most needed.

E.g. if no-one else is getting tested, then you're going to be the only one who got it wrong. You'd think there'd be prestige - logically and sensibly there would be - but when something has been missing for a long time, the basic channels through which ideas get considered and win or lose favour will rewire, into what we increasingly have, which is about ever elongating beauty contexts, media profile, getting a name for yourself. Without prediction, that's just the default drift of the stream and what you need most in an ever elongating process that may never conclude, is an idea that never goes away.

It's much more advanced in the multiverse community than yours, but if you want to know where it's going to be in a few years look there. Question: why is max tegmarks multiverse better than david deutsch's? Max Tegmark is sexier.

You get the same default in poorly performing corporate environments, and much elsewhere besides. It's no-ones fault and people can't be blamed for putting their families and career prospects first, and if the single standout way to get offered a better job is beauty contests, media savvy and lots of prose practice - which it is if there's no predictions - then people will do what they have to. But 'Science' cannot endure, even if prettier more eloquent scientists with higher profiles and sexy ideas can.

You say "It's no-ones fault (...)"; NO, it is everyone's fault, public included, and how to fix it is a question of dignity. If everyone resigns because everyone resigns, then civilization is dead. But each individual has his share and can realize this. Then action is possible: You decide for yourself what a high profile is; eventually, no-one else can do that for you - unless you resign.

1. What kind or precision would be possible with such a measurement? A few percent? A part per thousand? A part per million?

2. How much precision would be necessary to improve upon existing experimental limits from non-black hole binary systems?

3. Has anyone done the phenomenology to determine which classes of quantum gravity theories predict the biggest deviations from GR in a black hole-pulsar system, and which would be indistinguishable for all practical purposes? (Your response of 11:25 AM, July 12, 2016 to Uncle Al would suggest that the answer is basically "no", which is pretty stunning in and of itself given the century of work that has been devoted to GR and half century plus of pretty intense research that has been devoted to quantum gravity in particular.)

4. There are lots of areas of physics where the leading competing beyond the Standard Model and GR theories are well known and well defined, but I, at least, have a much more fuzzy sense of the main contenders in quantum gravity theories at a level of concreteness that would be applicable in analyzing this kind of system.

My intuition is that the big differences in phenomenological prediction are mostly within different approaches within the string theory and within loop quantum gravity camps respectively, rather than being mostly between those camps, when in comes to predictions about a black hole-pulsar system. For example, it seems like both camps have independently come up with similar conclusions about black hole entropy.

It seems like the big differences between the string theorists and the LQG folks phenomenologically involve differences in cosmology and in long range behavior arising from Lorentz invariance violations that might be present in LQG.

Is this intuition wrong or mostly wrong, or god forbid, "not even wrong"?

One cannot learn, not even roughly, what their idea is about from your blog-post. The most detailed information about the signal they propose to search for u give us is: "...the pulsar’s signal might scrape by the black hole and thus encode information about the horizon which we can catch on Earth." How is, what "information about the horizon" encoded?!

In the paper they actually assume an order one (!) fluctuation of the metric which, yes, has been suggested by some folks, but I can't say I'm very optimistic about this. In any case, it's something to look for. As you anticipated, the answer to your other questions is basically "nobody knows." These are difficult calculations and nobody has the time and/or funding to do them. There just isn't any support for this. It's totally possible today to spend your career philosophizing about black hole thermodynamics, but if you try to extract a prediction from that for a black hole merger, you need to make contact to an entirely different community and there is very little funding in this overlap region. Sad but true. Best,

The previous sentence refers to another phenomenon, there is no way to infer from your post that it also refers to the quantum BH horizon case. Can you explain in a few sentences for one hypothetical model how such fluctuations arise, on what spatio-temporal scale they might occur and what quantitative effect this would have on the pulsar signals? It would be best to add this to your post.

Have they seen any example of starlight from an ordinary star disappearing and reappearing when the star is eclipsed for a while by a BH. I understand there is Einstein lensing effect, but starlight going straight in the direction of BH should disappear into BH.

Kashyap: what you talk is a very hard measurement, since angular size of a BH is very small. The event horizon telescope is the firstwhich will try to "image" a black hole.Sabine: Thanks. Also I checked that link again and wanted to ask if you ever checked about whether the talks of space-time odyssey meeting are online?shantanu